The invention concerns a fiber optic transmission method having reduced sensitivity to chromatic dispersion caused by the fiber; a transmission device and system for implementing this method.
The maximal transmission distance over a repeaterless optical fiber is limited by attenuation and chromatic dispersion. Erbium-doped optical fiber amplifiers can correct the attenuation but the chromatic dispersion remains a major limitation in the case of signals having a high bit rate, for example 10 Gbit/s. Various prior art methods can compensate chromatic dispersion on a given fiber section, at least in part.
Other prior art methods are directed to making the transmission less sensitive to chromatic dispersion, without changing the latter. For example, the article "10 Gbit/s unrepeated three-level optical transmission over 100 km of standard fibre" by X. Gu and L. C. Blanck published in Electronics Letters, 9th Dec. 1993, vol. 29, n.degree. 25, describes a method consisting of:
encoding a data pattern to be transmitted in the form of a duobinary signal, i.e. a signal which can assume values approximately equal to +1, 0 and -1, by reducing the bandwidth of a binary signal; PA1 modulating an optical carrier with the values assumed by this signal, to obtain an amplitude modulated carrier at three different levels; PA1 transmitting the modulated carrier over an optical fiber without using any particular method of compensating chromatic dispersion; PA1 demodulating the optical signal received at the distant end of the optical fiber using a photoreceiver supplying an electrical signal representative of the amplitude of the received optical signal, and comparing this signal with two reference values to reconstitute two binary signals in parallel; and PA1 decoding the two binary signals to restore the data pattern. PA1 a data pattern is coded in the form of a ternary modulation signal which can assume three meaningful values: a maximal value, a minimal value and an intermediate value between the maximal value and the minimal value; and PA1 an optical carrier is modulated according to the values assumed by the modulation signal; PA1 characterized in that, to modulate said carrier, the carrier is transmitted with: PA1 the data pattern is coded in the form of a binary signal if it is not already in this form; PA1 this binary signal is coded in the form of another binary signal by differential coding; and PA1 this other binary signal is filtered to reduce its bandwidth and thereby obtain a ternary signal. PA1 means for coding a data pattern to be transmitted in the form of a ternary modulation signal which can assume three meaningful values: a minimal value, a maximal value and an intermediate value between the maximal value and the minimal value; and PA1 means for modulating an optical carrier according to the values assumed by the modulation signal; PA1 and characterized in that the optical carrier modulator means include a modulator supplying a modulated carrier having: PA1 means for coding said data pattern in the form of a binary signal if it is not already in this form; PA1 means for coding the binary signal in the form of another binary signal by differential coding; and PA1 means for filtering the other binary signal to reduce its bandwidth and thereby to obtain a ternary signal. PA1 means for coding said data pattern in the form of a ternary signal with three discrete levels if it is not already in this form; and PA1 optional means for filtering the ternary signal by reducing its bandwidth. PA1 a transmitter device for modulating an optical carrier with a data pattern; PA1 an optical transmission link one end of which is coupled to an output of the transmitter device; PA1 means for demodulating the modulated optical carrier having an input coupled to a second end of the link and supplying a signal representing the envelope of the power of the modulated optical carrier; and PA1 a comparator for comparing the signal to a reference value and deducing a binary signal representing the data pattern transmitted; PA1 characterized in that it includes a transmitter device in accordance with the invention.
The use of duobinary coding and amplitude modulation reduces the bandwidth occupied by the signal transmitted. This reduces the effect of chromatic dispersion and therefore significantly increases the maximal transmission distance. Another advantage is that the optical transmitter and optical receiver can have a narrower bandwidth than in the case of conventional transmission in which the carrier is amplitude modulated by a binary signal.
However, this prior art method has a drawback: it is necessary to discriminate three different amplitude levels in the electrical signal supplied by the photodetector. Discriminating three levels is much more difficult than discriminating two levels as required for amplitude modulation by a binary signal. Also, the signal/noise ratio is lower than that which would be obtained with a binary signal.